I didn't know that about PCs. So through software, Realtime overlaying can actually create a 2nd layer but requires a powerful CPU. What if you program it in assembly or machine code, will that make a difference? And what is the limit of it, how best would you use this on the PCE?

I saw Gradius2 L.2, impressive L.6 with its diagonal walls looks good to. Check out MealStorm L.3, it looks like 3 layers. And C-64's Enforcer2, and Katakis.